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Boxwood beware!

Editor’s note: This article is from the archives of the MSUCrop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The diagnostic lab has received several samples of boxwood this spring. These boxwood were growing along the edge of walkways and sidewalks in residential areas. The bushes reportedly looked great last fall, but this spring the bushes had extensive blighting and in some cases were dead. Needless to say the homeowners were extremely upset and were quick to find someone to blame for the damage. In this case, the blame was passed to the companies that had been hired to keep the sidewalks clear from snow and ice throughout the winter, salt damage was the suspected cause of the decline. As diagnosticians, we were contacted to determine what caused the damage. We begin by looking at symptoms on the bushes submitted, reviewing pictures of the site provided by the client and testing the soluble salts level of the soil submitted.

Declining boxwood shrubs planted along a residential sidewalk.

A soluble salt testing provides a quantified level of soil salinity. The higher the value the more saline in the soil. Soluble salts levels of the soil taken from these boxwood samples were as high as 1.3 mS/cm or 910 ppm (measured with a 1:2 dilution). The values in these soil samples were taken several months after the last salt application would have been applied, and the salt levels had been reduced by leaching. Salt is leached from soil with rain and irrigation water. There is no way to determine what the salt levels in this soil were earlier in the season.

Measuring the salt content is the easy part, interpreting the values is more complicated. The thresholds for tolerance and plant damage varies depending on the soil type, moisture content of the soil and the type of plant being grown in the soil. Salt sensitive crops are expected to have damage in field soil with a level of 1.00 or higher. Most crops are expected to have severe injury in dry soil where soluble salts levels are 1.5 or higher (Warncke, 2000). Depending on the references you consult, boxwood is classified as sensitive to moderately sensitive to salt damage.

Boxwood plantings along sidewalks can readily become a maintenance issue. Landscape designers should consider the salt sensitivity of plants placed next to sidewalks or other areas where salt is likely to be used. Many landscapers and lawn maintenance companies operate snow removal services during the winter. These professionals should be aware of the pitfalls associated with using deicing salts near boxwood and other salt-sensitive plants. Additionally, homeowners need to balance the need for safety by deicing the sidewalk with the potential damage salt exposure causes to adjacent plantings.